'91 was the first year of the 4L60 in the blazer. It's likely someone swapped a later tranny into the '88 truck if it was a 4L60. In any case a 4L60 is identical to a 700R4 which may be why it came up as a 4L60. The 4L60E was the later tranny which replaced the 700R4 and 4L60. It can be distinguished by the round vs square electrical plug. The 4L60E requires an electronic controller like the 4L80E. The 700R4 and 4L60 only require a device to lockup the torque converter which could be as simple as a toggle switch.
I called the tranny out of the '91 a 700R4 but it's code would come up as a 4L60 as '91 was the cutoff year. Bottom line, there is absolutely no difference between the two other than updated valve bodies, ports, some accumulators, and other nick nacks that progressed over time. They are completely interchangable.
Mike